1. Field of the Invention
The present invention relates to a sheet processing apparatus for receiving sheets, which are discharged from an image forming apparatus such as a copying machine, a printer, a facsimile, and a composite machine, or discharged from other business machines, and stacking the sheets on ascendable/descendable sheet stacking means.
2. Description of the Related Art
A sheet processing apparatus for receiving sheets discharged from an image forming apparatus such as a copying machine, a printer, and a facsimile to perform processes such as alignment, sorting, stacking, stapling, bookbinding, punching, and checking has been put into practical use. In addition, in some types of image forming apparatuses, such the sheet processing apparatus is built therein or connected thereto as a so-called purchase option.
Some types of sheet processing apparatuses are provided with an ascendable/descendable stacking tray, and are capable of continuously stacking a large amount of sheets by allowing the stacking tray to descend in accordance with a stacking process of sheets or sheet bundles.
A sheet processing apparatus disclosed JP H10-198101 A is provided with an ascendable/descendable stacking tray having a plurality of stages, and is capable of stacking sheets after positioning a designated stage of the stacking tray having the plurality of stages at a sheet receiving position. In addition, an arithmetic control device of the sheet processing apparatus estimates a total estimated time for sheet processing prior to performing the sheet processing, and notifies an image forming apparatus of a sheet discharge interval (i.e., time) calculated on a predetermined safety time or a safety factor in addition to the total estimated time. Further, the arithmetic control device of the image forming apparatus allows sheets to undergo image formation at the sheet discharge intervals and allows the image forming apparatus to discharge the sheets.
The total estimated time in the sheet processing apparatus is constituted of individual estimated times for respective operations from sheet reception to completion of height adjustment of a stacking tray on which sheets are already stacked, such as a sheet transporting time, a staple process time, and a stacking tray-moving time. In many cases, a length of each of the individual estimated times is 1 second or shorter. Meanwhile, the individual estimated time for an operation of switching a stacking tray of a discharge destination is incomparably long, that is, from 10 seconds to 30 seconds. Accordingly, when the operation of switching the stacking tray is frequently performed, a waiting time for processing of the image forming apparatus increases, thereby remarkably lowering a process speed. As a result, both the sheet processing apparatus and the image forming apparatus cannot utilize potential processing abilities of those.